Metal fluorides such as aluminium fluoride (AlF3) and magnesium fluoride (MgF2) are used as low-refractive index materials for antireflective coatings for optical elements such as lenses and mirrors with visible light.
A sputtering process is superior to vacuum vapor deposition processes in terms of reproducibility, the control of film thickness distribution, low-temperature deposition, and the like and therefore is attracting attention as a method of forming an antireflective coating (thin film) containing a metal fluoride. The sputtering process uses plasma particles such as charged particles and ejects atoms from a material to form a film. In the case of depositing a metal fluoride, it is difficult for the sputtering process to control the reactivity of fluorine with a sputtering material or a base material on which a film is formed and the damage of a substrate or a film by charged particles because of these features. In the case of forming an antireflective coating containing a metal fluoride on a base material of a glass substrate or a metal oxide film, fluorine atoms, which have strong oxidizing properties, enter a base interface and therefore the base material of the glass substrate or the metal oxide film is reduced to form an altered layer with electronic defects. Therefore, there is a problem in that absorption occurs at a wavelength longer than the wavelength corresponding to the band gap.
PTL 1 discloses an optical thin film in which a metal oxide thin film containing at least one of SiO2, ZrO2, and Al2O3 is placed between a glass substrate and a metal fluoride film (MgF2 film) for the purpose of suppressing light absorption occurring mainly at the boundary between the glass substrate and a film and also discloses a method of manufacturing the optical thin film.
An optical thin film including a metal oxide thin film which is placed between a base, such as a glass substrate, made of a metal oxide material and a metal fluoride film as disclosed in PTL 1 and which contains at least one of SiO2, ZrO2, and Al2O3 can be used to suppress light absorption occurring mainly at the boundary between the glass substrate and a film. However, available refractive index is limited to 1.46 (SiO2), 1.63 (Al2O3) and 2.05 (ZrO2). Furthermore, in the case of forming a metal oxide thin film containing SiO2 between a substrate and a metal fluoride film, there is a problem in that the metal fluoride thin film is likely to be peeled from the metal oxide film.